Environment-proof limit switch



L. J. BULLIET Jan. 16, 1968 ENVIRONMENT, PROOF LIMIT SWITCH 4Sheets-Sheet 1 Filed Aug. 20, 1965 (ENao mvzzmoa Leander J. Bullief,

T RNE ,1968 L. J. BULLIET ENVIRONMENT, PROOF LIMIT SWITCH 4 Sheets-Sheet2 Filed Aug. 20, 1965 'FIG.5

L. J. BU LLIET ENVIRONMENT, PROOF LIMIT SWITCH 4 Sheets-Sheet 5 FiledAug. 20, 1965 FIG. 8

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ENVIRONMENT, PROOF LIMIT SWITCH Filed Aug. 20, 1965 4 Sheets-Sheet 4-FIG. 14

.2 16A FIG.12 FIG1 A United States Patent 3,364,318 ENVIRGNMENT-PROOFLIMIT SWITCH Leander .I. Bulliet, Rockford, Ill., assignor to TheBahcock & Wilcox Company, New York, N.Y., a corporation of New JerseyFiled Aug. 20, I965, Ser. No. 481,239 Claims. (Cl. 200-47) Thisinvention pertains to a novel switch, and more particularly to a limitswitch constructed for highly accurate reproductive operation andparticularly immune to malfunction due to the presence of a wide varietyof adverse environmental conditions.

Limit switches have been characterized by limitations in many types ofservice, particularly when subject to the presence of certain liquid,vaporous or dust-like contaminants of either electrically conductive orelectrically insulating nature. Under such circumstances thereproductive accuracy and the ultimate life of the switch has beenunsatisfactory. Many attempts have been made to overcome themalfunctions of limit switches under diflicult' environment conditions,but all reasonably successful constructions have been prohibitivelyexpensive, and thus generally impractical.

In the present invention, a limit switch is provided which is generallyimmune to the environmental difiiculties encountered in service. This isaccomplished by utilizing a magnetic type switch with the electricalcontacts thereof completely and permanently sealed against contaminationby foreign materials. Moreover the external electrical connections ofthe limit switch are sealed after installation so that they can easilybe unsealed for removal or replace ment of a mechanically worn ordamaged switching component. The novel switch is further provided with asuitably rugged housing or enclosure to protect the electrical elementsagainst mechanical damage and relatively large contaminants such a chipsand shavings or the like. The switch is characterized by a high degreeof reproductive accuracy of operation through an extended life, ease ofinstallation without excessive manufacturing expenses.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages and specific object attained by its use,reference should be had to the accompanying drawings and descriptivematter in which there is illustrated and described a preferredembodiment of the invention.

Of the drawings:

FIG. 1 is a front view of one preferred embodiment of the invention;

FIG. 2 is a side view of the encapsulated electrical elements of FIG. 1;

FIG. 3 is a bottom view of the exterior only of the' electricalencapsulation of FIG. 2;

FIG. 4 is a rear view of the assembly shown in FIG. 1;

FIG. 5 is a sectional side view taken substantially along the sectionline 5-5 of FIG. 4;

FIG. 6 is an enlarged fragmentary view taken as indicated by 66 in FIG.5;

FIG. 7 is an exploded isometric view of some of the parts shown in FIGS.4 and 5;

FIG. 8 is a front view of a modified assembly embodying the invention;

FIG. 9 is a side view of the encapsulated parts of FIG. 8;

FIG. 10 is a bottom view of the outside only of the encapsulation shownin FIG. 9;

FIG. 11 is an isometric view, partly in section, of the encapsulation ofFIG. 9;

FIG. 12 is an electrical circuit diagram pertaining to electrical partsshown in FIG. 8;

FIG. 12A is an alternate showing of a portion of FIG. 12;

FIG. 13 is a rear view of the assembly of FIG. 8; and

FIG. 14 is a sectional side view taken substantially along 1414 of FIG.13.

In FIG. 1, numeral 2 indicates a switching element known in the art as areed switch. Reed switch 2 includes two flexible ferromagnetic leaves orreeds 4 and 6 which are sealed into opposite ends, respectively, of aglass envelope 8. The reeds 4 and 6 are of such length that they overlapaxially at about the longitudinal center of the glass envelope. Thereeds 4 and 6 are formed fiat throughout the area of overlap and forsome distance beyond this area. However, at the ends, these reeds arecircular in cross-section for facility in sealing the glass envelope.

In the area of overlap, the reeds 4 and 6 are overlaid with a thin filmof highly conductive material (usually gold) which assures goodconductivity when the reeds are in contact one with the other. Beforefinal sealing, the envelope 8 is evacuated and the air is replaced witha gas which is conductive to eflicient arc quenching and does notpromote oxidation or other chemical contamination of the contactingsurfaces.

As seen in FIG. 1, the overlap portions of reeds 4 and 6 are in contactwith one another. This is due to the presence of the permanent magnet 10in relatively close proximity to the reed switch. The poles of magnet 10are disposed on either side of the area of overlap of the reeds so thata portion of the magnetic lines of force are conducted through the reedsand the area of contact thereof. This magnetic force holds the reeds inmutual contact.

When the magnet is shifted to the left, as shown by the dotted lines inFIG. 1, the poles of magnet 10 no longer embrace the overlap area of thereeds. In this latter case, there are no lines of force tending to holdthe reeds in contact.

In the original sealing process, the reeds are so positioned that thereis a gap between them in the overlap area. When magnet 10 is placed inthe solid line position of FIG. 1, magnetic lines of force appear acrossthe gap between the reeds and create an attractive force. The reedsyield to this force and the contact is established by slight bending ofthe reeds. When magnet 10 is moved from its effective position, thereeds spring apart reestablishing the original gap.

Thus, if magnet 10 is moved back and forth between its solid and dottedline positions (FIG. 1), the contact between reeds 4 and 6 isalternately established and broken.

Shown also in FIG. 1 is a second reed switch 12. Reed switch 12 islocated behind switch 2 in FIG. 1 and so is shown partly broken away.The position of switch 12 is such that the magnet 10 is effective toclose its contacts when in the dotted line position. It is to beunderstood that the magnet 10 has sufiicient depth (perpendicular to theplane of FIG. 1) to be effective on the reeds of both reed switch 2 and12.

It is now clear that moving magnet 10 back and forth between its solidand dotted line positions results in alternate opening and closing ofthe contacts in switching elements 2 and 12. In this exemplaryarrangement, one contact is open when the other is closed and viceversa.

Several modifications of the switching element arrangement are possibleas will be obvious to those skilled in the art. For example, one reedswitch could be omitted if only a single throw switch is required. As afurther example, both reed switches could be oriented such that both areaffected by the magnet when in the same position. This would give twosets of contacts both open or closed at the same time. By increasing thedepth of magnet lil, additional reed switches could be accommodated;giving more than two sets of contacts arranged in any desiredcombination of opened and closed conditions. A still furthermodification could be effected by locating one or more reed switches soas to be affected by a position of magnet intermediate between the solidand dotted line positions. In the latter case, care must be exercised toassure that magnet 10 moves far enough away from one circuit closingposition to open that circuit when closing the next. This is a mattermerely of dimensions and of the magnet moving mechanism yet to bedescribed. Finally, reed switches are commercially available whichcontain more than one pair of contacts within the envelope. Use of suchreed switches present further possibilities for contact combinations;all within the spirit of the invention.

Persons skilled in the art will recognize that other specificarrangements of the magnetic circuit can be devised for opening andclosing the reed switch contacts in response to movement of a mechanicalmember. For example, one or more permanent magnets can be placed instationary relationship to the switch gap or gaps. In that case, themoving member would be a ferromagnetic piece which would act to shuntthe magnetic influence from the switch or to conduct that influence tothe switch or to perform both functions alternately.

For simplicity and definiteness, subsequent portions of thisspecification describe the one normally open and one normally closedcontact configuration which is most common in the limit switch art. Itis also confined to the moving magnet version of actuation.

Use of reed switches inherently provides complete isolation of thecontact gap from all exterior contamination. However, the external leadsextending from the ends of the envelope must be protected fromconductive contaminants and must also be provided with convenientterminals for connection to external wiring.

In FIG. 1 the reed switches 2 and 12 are shown completely embedded orencapsulated in a shaped or molded block 14 of electrically insulatingmaterial which should be impervious to any expected liquid or vaporouscontaminant. For convenience of disclosure only, this material is alsoassumed to be transparent in FIGS. 1 and 2. (In FIG. 2 the magnet andassociated mechanism are omitted.) Block 14 is secured to the housing byscrews 15 threaded into the housing.

In FIG. 1 are shown four terminal screws 16, 18, Ztl and 22. As shown inFIG. 2 (for screws and 22), each of these terminal screws is threadedinto a metal insert such as 24 and 26 molded into the block 14. Theinserts 24 and 26 are shown fluted or knurled on their externalcylindrical surfaces for secure holding against rotation in the moldedblock.

Each terminal screw is surrounded by a square (or other shape) cavity28, 3t 32 and 34 from the front surface of block 14 to a depth flushwith the front surface of the respective insert such as 24. Leadingdownward (FIG. 1) from each of the aforesaid cavities is a groove orwire channel 36, 38, 4t) and 42. Each of these grooves is of the samedepth as the corresponding screw cavity and leads through the bottomwall of block 14. The grooves are also shown in FIG. 3 which is a bottomview of block 14 but here treated as opaque to avoid confusing details.

The end leads of reed switch 2; are connected by condoctors 44 and 46 tothe inserts associated with screws A 16 and 26, respectively. Likewise,reed switch 12 is connected by conductors 48 and 54 to the inserts ofscrews 18 and 22;, respectively. Connections of these conductors may bemade by soldering, brazing or welding, etc. Referring to FIGS. 1 and 2,it is seen that the connections from the reed switches to the terminalscrews are completely enclosed in the molded block 14 and are thusshielded from environmental conditions.

As so far described, the terminal screws are exposed to the atmospherewithin the limit switch enclosure (yet to be described). Depending uponthe environment of application, the mentioned exposure may or may not beacceptable. Assuming that the only contaminant likely to invade thehousing is of a non-conducting kind (most oils, for example), theexposure of the terminal screws would'not be objectionable.

If electrically conductive dust, liquids or vapors were present in theenvironment, the exposed terminal screws would be subject to shortcircuiting or connecting to the enclosure (grounding) or both.

In the past, much effort has been expended to devise limit switchenclosures which will prevent entrance of dust, liquid and vaporouscontaminants. This effort has, of course, been to shield both thecontact gap and the terminals from exposure. In general, these effortshave led to both inconvenience of application and expensive sealingdevices. As the above description shows, the present invention isolatesthe most vulnerable elements, the contact gaps, by encapsulation.Shielding of the terminal screws then becomes relatively easy as willnow be explained.

External wires to the terminal screws are laid in the grooves 36, 38,4t) and 42. In preparing the external wires, care is exercised to leaveinsulation on the wire up to as near as practical to the terminalscrews. Thus, the portion of a wire lying in a groove is mostly orentirely covered with wire installation. After the external wires areinstalled under the terminal screws, the cavities 28, 3t), 32 and 34 andthe grooves 36, 38, 40 and 42 are filled with any of several availablesealing compounds. Choice of the particular sealing compound may be leftto the user. However, there are several characteristics which thecompound should have in most cases. It should be chemically resistant toexpected contaminants. It should adhere well to the wire insulation andalso to the material of block 14. This will assure complete sealing ofthe terminal screw from the atmosphere. Where occasional replacement ofthe switch, due to mechanical damage or wear, is anticipated, a compoundshould be chosen which will remain relatively soft compared to thematerial of block 14. This will permit removal of the sealant by meansof a sharp instrument without damaging the general encapsulation. Onecompound which is useful in a wide variety of conditions is siliconerubber cement.

It is now necessary to describe preferred means whereby the magnet It}is transferred from one of its contact closing positions to the other.This transfer is understood to be desired in response to the position ofsome moving machine member.

If the magnet 10 is moved directly by the moving member, it is foundthat extraordinary care must be exercised in guiding the moving memberif the switch is to change state (open or close) at exactly the sameposition of the moving member each time. For example, slight variationin the perpendicular distance from the magnet poles to the reeds willcause significant differences in the longitudinal position for contactaction.

In the present invention, the position of the magnet is made noncriticalby using a mechanical snap-action for original response to the machinemember position. When the snap-action mechanism trips, the magnet istransferred substantially instantaneously from a fully active to a fullyinactive position relative to an associated reed switch, or vice versa.

A great variety of snap-action mechanisms have been devised both in thelimit switch art and in many other mechanical fields. In the limitswitch prior art, snap actions have been used to transfer movingelectrical contacts quickly in response to moving machine partpositions. For purposes of the present invention, the magnet is snappedfrom place to place instead of a moving electrical contact.

The particular snap mechanism to be described is one of several whichhave found wide commercial acceptance in limit switch manufacture. Thisone is readily adaptable to the purposes of the invention but is not perse a part of the invention. On the other hand, the invention can bepracticed by adaptation of other specific snap mechanisms.

Considering FIG. 1 as a front view of the limit switch, FIG. 4 is a rearview.

Referring to FIG. 4, numeral 52 designates a metal housing (usually adie casting). Journalled in this housing, as best seen in FIG. 5, is amain actuating shaft 54. Press fitted to one end of shaft 54 is thedriver arm 56. The opposite end of shaft 54 is fitted to one end of theposition sensing arm 58.

The end of shaft 54 which carries arm 58 is drilled part way through,longitudinally to form a cavity 60 which is taper threaded internally.Into the walls of cavity 60 is cut a slot 62 which is shown in FIG. 5and is better seen in FIGS. 1 and 6. The outer periphery of the portionof shaft 54 which enters arm 58 has fine longitudinal serrations 64which do not show in the drawings except in the enlarged fragmentaryview, FIG. 6. The mating hole in arm 58 is internally serrated tocomplement the serrations on shaft 54. By means of the serrations, thearm 58 can be fitted on shaft 54 in any angular relation consistent withthe pitch of the serrations. When a suitable angle for the arm 58 hasbeen chosen, it can be made secure by tightening the tapered andexternally threaded plug 66 (FIGS. 1 and 6) which is threaded into thecavity 60. As the plug is tightened, its taper spreads the slot 62slightly thereby holding arm 58 in place.

The end of arm 58, opposite the shaft 54, is slotted to form a forkwhich receives the roller 68. Roller 68 is free to rotate about a pin 70press-fitted in a hole through the fork side members of arm 58.

The purpose of arm 58 is to detect the position of a moving machinemember and transmit it to the inner mechanism of the limit switch. Thispurpose is indicated schematically by the cam 72 shown in FIG. 4. Cam 72may be assumed to be attached to the machine member (not shown) which ismovable as indicated by the double headed arrow. If the cam 72 moves tothe left, it will eventually make contact with the roller 68; moving arm58, shaft 54 and arm 56 counterclockwise as seen in the figure. As willappear, below, there is a limit to the counterclockwise motion of arm 56without damage to the mechanism. In an actual device, spring means areincorporated in the hub portion of arm 56 to permit further rotation(overtravel) of shaft 54 after motion of arm 56 is blocked. Since theovertravel provision is only incidental to the practical application ofthe device and is not part of the invention, it is not described here.

As seen in FIG. 4, the lower end of arm 56' is normally held at theleftward limit of its travel by the compression spring 74. One end ofspring 74 bears against a side of arm 56. The other end of spring 74bears against a portion of the housing 52 defined by a recess 76. In thenormal position shown, spring 74 is partially compressed to maintain arm56 firmly at its clockwise limit of rotation.

Clockwise rotation of arm 56 is limited by contact with a portion of thetrigger member 78 which is itself restrained against leftward motion bycontact with the abutment 79 cast integral with the housing 5'2.

With arm 56 in the position described, a roller 80 bears upon the uppersurface of the rocker arm 82 which is press-fitted or pinned to an endof the snap-action shaft 84. The roller is freely rotatable about a pinshaft 86 which is press-fitted in two spaced apart fork portions of asliding guide block 88. Forceful contact between roller 80 and rocker 82is maintained by partially compressed coil spring 90. One end of springbears upon the upper end of block 88 while the other end bears upon thebottom of a blind-drilled axial hole 92 in arm 56. The relation of arm56, spring 90, block 88 and roller 80 is more specifically shown in theisometric exploded view, FIG. 7.

As shown in FIG. 4, the force of springe' 90 is exerted, through roller80, upon rocker 82 to the left of the axis of shaft 84. This tends tocause counterclockwise rotation of shaft 84. As seen in FIG. 5, shaft 84extends through and is rotatable in the wall 92 and a boss 94 of thehousing 52. At the end of shaft 84 opposite rocker 82 is pressfitted orpinned a non-ferromagnetic arm 96 which partakes of any rotation ofshaft 84. In FIG. 1, the arm 96 is shown limited in clockwise rotationby contact with a pin 98 which is press fitted into a boss 100 eastintegral with the wall 92 of housing 52. The clockwise limit of arm 96in FIG. 1 corresponds with the counterclockwise limit of rocker 82 ofFIG. 4 because of the front-to-back relation of these views. Thus, thecounterclockwise r0- tation of rocker 82 in FIG. 4 is limited by arm 96and pin 98 in FIG. 1. As further seen in FIG. 1, the earlier describedmagnet 10 is fixed to the upper end of arm 96 by cementing or otherwise.The non-ferromagnetic composition of arm 96 prevents shunting the fieldof magnet 10.

If now the shaft 54 is turned counterclockwise (FIG. 4), due to actionof cam 72 on arm 58 or otherwise, the lower end of arm 56 will movetoward the right; further compressing spring 74. As arm 56 moves, theroller 80 moves upward along the slope of the upper surface of rocker82; further compressing spring 90. As the line of force of spring 90reaches and passes coincidence with the axis of shaft 84, the torque onthat shaft changes from counterclockwise to clockwise. Thereafter,rotation of shaft 84 is prevented by engagement of the right hand end ofrocker 82 against the lower surface of a notch 102 in trigger 104. Atthe time of reversal of torque on shaft 84, force between arm 96 and pin98 is relieved (trans ferred to rocker 82 and trigger 104, asaforesaid). During this transition, there may be slight rotation ofshaft 84 due to deflection of pin 98 or clearance at notch 102 or both.However, such motion is not sufficient to disturb the previouslydescribed function of magnet 10 in maintaining electrical contactbetween the reeds 4 and 6.

As the lower end of arm 56 (FIG. 4) continues its motion to the right,it eventually engages an upper end portion of trigger 104, whereuponthat trigger is forced to rotate clockwise about a pivot pin 106 fixedin a boss 108 integral with wall 92 of the housing. As trigger 104 movesas described, it compresses return spring 110 based in a cavity 112 ofthe housing.

As the above described related motions of arm 56 and trigger 104continue, the compression of spring 90 becomes greater and its point ofaction moves farther to the right of the axis of shaft 84. Consequently,there is a build-up of restrained clockwise torque relative to the axisof shaft 84. At a definite point in this action, the lower surface ofnotch 102 leaves the underside of rocker 82. Precisely then, therestraint on the aforesaid torque is removed allowing spring 90 torotate the rocker 82 and the shaft 84 suddenly clockwise (FIG. 4). InFIG. 1, the above mentioned motion appears as a sudden snap of lever 96counterclockwise into contact with stop pin 114 fixed in boss 116integral with wall 92. This transferred position is shown by dottedlines in FIG. 1. Obviously, the snap action of lever 96 carries themagnet 10 with it; resulting in the transfer of electrical conditions inthe reed switches which was described earlier.

During the motion of arm 56 to the right (FIG. 4), contact with trigger78 was relieved but the trigger was,

at first, maintained in substantially its original position byinterference by the left end of rocker 82. When the rocker suddenlytransferred, this interference was removed by the left end of rocker 82moving above the lower surface of notch 118 in trigger 78. When thisoccurred, the spring 129 forced the trigger 78 clockwise sufficientlyfor the notch 118 to restrain rocker 32 on a subsequent reverse snap.

After the snap action, the various parts of the snap mechanism aredisposed substantially as shown in FIG. 13 (except for obviousdifferences in dimensions).

When the original operating force is removed, spring 74 will return arm56 to the original position shown in FIG. 4. During this return motion,snap action takes place in reverse to that described. That is, rocker 82is held in its previously operated position until arm 56 moves trigger78 aside.

Arm 56 can be held normally in its opposite operative position byplacing spring 74 on the other side of arm 56 and having it bear on theleft wall of cavity 122. Spring 74- may also be removed entirely if itis desirable to actuate arm 58 in either direction by mechanical meansand have it remain so until mechanically actuated in the oppositedirection. In the absence of spring 74, arm 56 is maintained in eitheroperated position by detent action of spring 90 forcing roller 30 downthe slope of rocker 32.

In FIGS. 1 and is shown a cover plate 124 which is omitted from FIG. 4.In plate 124 is a plurality of countersunk screw holes 126 so spaced asto match with threaded holes 128 in the walls of housing 52 (see FIG.4). By use of flat headed machine screws, the plate 124 can be attachedto the housing to cover the mechanism shown in FIG. 4. The use of flatheaded screws countersunk into the cover plate leaves a flat surface(back of FIG. 1; to the left in FIG. 5) for mounting the device.Mounting is further provided for by mounting-screw holes 130 throughplate 124. Threaded holes 132 are provided in the housing to receivescrews for holding any cover desired to protect the parts shown in FIG.1 from mechanical damage.

At 134- (FIGS. 1, 4 and 5) is a cast hub or boss, integral with thehousing. Through this boss is a hole 136 which is provided with taperedthreads for receiving a standard electrical conduit fitting.

When installing the back plate 124 and/or a front cover plate, it may bedesirable to include some kind of gasket between the cover and thehousing wall to exclude liquids, and dust. However, such precaution isnot nearly so vital as in conventional limit switch practice since, aspreviously emphasized, intruding foreign matter cannot reach anyelectrical conducting parts. One reason for gasketing might be thepresence of sticky or abrasive contaminants which could damage themechanical parts. The same considerations render unnecessary the carefulsealing of conduit opening 136 or the bearing of shaft 54. In short, anyclosure of openings in the housing need provide only for protectionagainst gross invasion of foreign matter and against mechanical damage.It is clear that the mere provision of a general housing is sufficientto protect the magnetic circuit of magnet against adverse effects offerrous chips or shavings. The active magnetic field gap is so smallcompared to dimensions of the housing that any chips resting on thehousing cannot adversely affect the desired magnetic function.

All reed switches are generally considered to be in th miniature classof electrical components. Even so, the lengths of the larger of theseswitches are such as to make the limit switch assembly so far describedsomewhat larger than is desirable for many applications. Furthermore,the largest standard reed switches have limited current carryingcapacity. A limit switch made according to FIGS. 1 through 7, would belarger in dimensions but less in load handling capacity than many typesof conventional limit switches. In spite of these facts, the devicedescribed is 8 not without utility where the mentioned limitations arenot critical.

Now to be described is a modification of the invention which providesmore compact construction along with greater load handling capacity.

In the figures to be described, analogous parts are given the samenumerals as in the previous figures but with the sufiix a added. In thiscontext, analogous means substantially identical function but withpossible difference in specific form or dimension. Non analogous partsare given new numerals.

In FIG. 8, numerals 2a and 12a refer to microminiature reed switches.These switches are of the same configuration and general construction asreed switches 2 and 12 except that they are considerably less than onehalf the size in each dimension. These smaller reed switches obviouslypermit a smaller over all assembly. However, they,

necessarily have much less current carrying capacity. As will appearbelow, it is possible to take advantage of the smaller dimensions andyet include small amplifyin devices to permit ultimate current switchingcapacity greater than that of the larger reed switches alone.

As seen in FIG. 8, the reed switches 2a and 12a are molded in aninsulating block 14a in such position as to be operatively affected bythe field of magnet 10a. The magnet is attached to a non-ferromagneticarm 96a fixed on shaft 84a. In one rest position, arm 96a is heldagainst the abutment stop 138 which is molded integral with the block14a. In this position of magnet 10, switch 12a is held in electricallyconductive condition and switch 2a i non-conductive. Upon actuation ofthe snap mechanism, the arm 96a shifts quickly to contact with stopabutment 140 also molded as a part of block 14a. In this second restposition, magnet 10a is effective to render switch 2a electricallyconductive and switch 12a non-conductive.

Several types of semi-conductor devices are available which may beswitched from a substantially non-conductive to a substantially highlyconductive (very high impedance to very low impedance) by means of aswitch carrying very low current. Such devices include switchingtransistors, silicon controlled rectifiers, gate-controlled A.C. powerswitches, etc. With the additional complication of a filament currentsource, vacuum tubes could also be adapted to this purpose. Theinvention is meant to include any suitable means for using a low currentcapacity switch to control the flow of a much larger current in asubstantially on-and-otf manner.

Both for purposes of explanation and for appropriate constructioncharacteristics, the preferred amplifier at this time is thegate-controlled A.C. power switch. This semiconductor device is known inthe trade as a Triac and will be so referred to hereinafter. Onedesirable feature of the Triac is that it switches an A.C. load with buta single control terminal. This is an advantage because the greatmajority of limit switch applications involve A.C. loads. In case of aD.C. load, a different amplifier would be preferred. In common with mostpower range semiconductor devices, the load current capacity of theTriac is many times the control current required. Specifically, theTriac can switch a load of several amperes with a few milliamperes ofcontrol circuit current.

For simplicity, electrical connections are not shown in FIGS. 8 and 9.FIGURE 12 is a schematic circuit diagram for one of the two switchingchannels; namely, the one associated with reed switch 2a.

In FIG. 12, numeral 142 designates a Triac which is provided with a loadcircuit terminal 144 and a gate or control circuit terminal 146. Asecond load circuit connection is, made to the Triac by soldering orotherwise connecting a conductor to the case of the device as at 148.The two load-circuit connections 144 and 148 are extended by electricalconductors to the external terminal screws 16a and 20a, respectively.

As shown by broken lines in FIG. 12, terminal screw 29a (see FIG. 8also) is to be connected to one side of an electrical load which maytypically be the coil of an AC. operated electromechanical relay. Theother side of this load is connected to one side of an A.C. supplysource of voltage and frequency appropriate to the load. The remainingside of the power supply is connected to terminal screw 1611. Thus, thesubstance of the Triac between connections 144 and 148 is analogous to asimple'switch. That is, when the impedance between these connections isvery high, it corresponds to an open switch and the load is essentiallydeenergized. When this impedance is nearly zero, it corresponds to aclosed switch and the load is energized. The performance of the Triacmust be regarded as analogous rather than equivalent to a switch becauseits internal impedance is never completely zero or infinite.

For the purpose of changing the impedance of the load current pathwithin the Triac, the gate or control terminal 146 is connected to oneend of resistor 150. The other end of this resistor is connected to oneside of switch 2a. The opposite side of switch is connected to the caseconnection 148 of the Triac 142. The characteristics of the Triac aresuch that a closed circuit condition of switch 2a results in lowimpedance in the load path of the Triac. Conversely, an open circuit atswitch 20 produces a substantially open circuit through the Triac to theload.

When switch 2a is closed, there is a flow of current through this switchwhich is limited in amount mainly by the value of resistor 150. Ifresistor 150 is of too large a value, the flow of current through thecontrol channel in the Triac will not be suflicient to reduce the loadcircuit impedance adequately. However, experience has shown thatresistor 150 may be so chosen as to produce positive switching of theload circuit without exceeding a conservative current rating for themicrominiature reed switch 2a.

In FIG. 12A, Triac 142 is represented by its proposed graphic symbolinstead of the pictorial showing used in FIG. 12.. The circuit diagramof FIG. 12 may be modified by substituting this symbol and making theconnections indicated by the repeated numerals.

It is to be understood that a separate Triac 152 (FIG. 8) is provided tobe controlled by reed switch 1201. The circuit for Triac 152 would beelectrically identical to FIG. 12; a separate gate circuit resistorbeing used in place of resistor 150 and the external connections beingmade to terminal screws 18a and 22a (FIG. 8).

As shown in FIGS. 8 and 9, the Triacs are completely encapsulated in theinsulating block 14a. This construction is necessary to prevent accessof foreign matter to the Triacs. However, the encapsulation does impairthe escape of heat from the Triacs. Since semiconductor devices arelimited in their current ratings by the heat dissipation characteristicsof their environment, this encapsulation places a load currentrestriction on the Triac ratings. However, the normal load ratings ofcommercial Triacs are so high that this derating still leaves a loadcurrent capability considerably in excess of that of the larger reedswitches shown in FIG. 1. Specifically, Triacs 142 and 152 may readilybe used for controlling the coils of conventional relays regularly usedin machine tool control circuitry.

An alternate construction can obviously be provided by omitting theTriacs from the housing of the limit switch and connecting the leads ofswitches 2a and 12a to the terminals 16a and 20a for one switch and to18a and 22a for the other. In that case, the Triacs would be locatedremotely in a separate enclosure such as the general control cabinet ofthe machine tool. The latter arrangement would permit using full currentratings of the Triacs but would lose the advantage of self-containmentas shown in FIG. 8.

Means for sealing the external connections in FIG. 8 are generallysimilar to those described in reference to FIG. 1, but they aredifferent in detail because of the more compact arrangement. As before,each terminal screw is located in a cavity with a narrow wire grooveleading to the bottom of the molded encapsulating block. However, asshown in FIG. 9, the adjacent terminal in serts 24a and 26a (likewisefor the inserts under screws 16a and 18a) are placed at ditferent levelsin block 14a. By this expedient, the wire leading to screw 20a can passover screw 22a. Furthermore, the groove 42a for the wire leading toscrew 20a has a floor in it above the level of screw 22a. By pullingthis upper wire reasonably taut, a space is assured for the entry ofsealing compound between the screw 22a and the wire to screw 20a. Theserelations are further emphasized in FIG. 10 and in the partially cutawayisometric view in FIG. 11.

Mechanism for snapping lever 96:: from one switch closing position tothe other is shown in FIGS. 13 and 14. This mechanism is functionallyidentical to the snap mechanism described in reference to FIG. 4, 5, 6and 7. The only differences are in dimensions, some of which are madesmaller in the more compact assembly. The housing and cover plates arealso similar to those previously described except for dimensions andsome minor changes in configuration. The principles of protectionagainst environmental contaminants are exactly as previously described.

What is claimed is:

1. A limit switch for controlling an electrical circuit comprising amolded block of electrically insulating material, a member moveablebetween limits within said molded block, snap-action means forpositioning said member in any one of a plurality of rest positions, amagnetic member on said member, electrical switch means encapsulatedwithin said molded block in proximity to and operable by said magneticmember, electrical terminals within said molded block, electricconnections imbedded in said molded block connecting said switch meanswith said electric terminals, and means sealing said electricalconnections and electrical connections between said terminals and anexternal electric circuit to be controlled by said switch.

2. A limit switch for controlling an electric circuit comprising a blockof electrically insulating material, an arm mounted at one end on arotatable shaft and moveable between limits within said block,snap-action means attached to said rotatable shaft for abruptlypositioning said arm in any one of a plurality of rest positions, amagnetic member on the other end of said arm, an electrical switchelement encapsulated within said block in proximity to and operable 'bysaid magnetic member, means sealing the electrical connections withinsaid switch and between said switch and an electric circuit to becontrolled, and a member having a portion extending beyond said moldedblock operably connected with said snap-action means to actuate saidlimit switch.

3. A limit switch comprising a block of electrically insulatingmaterial, an arm moveable between limits within said block, snap-actionmeans for positioning said arm in any one of a plurality of restpositions, magnetic means activated by movement of said arm, anelectrical switch element encapsulated within said block in proximity toand operable by said magnetic means, terminal screws mounted withincorresponding recesses within said block, wire means im'bedded withinsaid block and connecting said switch element with said terminal screws,means defining grooves in said block between the recesses of saidterminal screws and the exterior of said block,

' said grooves facilitating the insertion of electrical sealing materialin said recesses and grooves when connecting said switch with anelectrical circuit actuated by said switch, and housing means enclosingsaid limit switch.

4. A limit switch according to claim 1 wherein electrical valving meansregulates electric flow therethrough responsive to actuation of saidelectrical switch means, said valving means being encapsulated withinsaid block and electrically connected with said electrical switch means.

3,364,318 1 1 1 2 5. A limit switch according to claim 4 wherein saidOTHER REFERENCES electrical valving means comprises a Triac.

Lowry, Voltage Regulated SCR Inverter With Sine R ferences Cit d WaveOutput' and Current Limiting, General Electric r Application Note, June1961, p. 1 UNITED STATES PATENTS Rudisill et a1, Guide to LimitSwitches, Product 3,031,385 3/1963 Scott 200-47 Engineering, Nov. 12,1962, p. 99. 3,198,902 8/1965 Deshautreaux 335207 3,215,864 11/1965Doyle et al. 307-885 BERNARD A. GlLHEANY, Primary Examiner. 3,247,3424/1966 Ott et a1. 335-207 2,795,678 6/1957 Mertler 200-168 0

1. A LIMIT SWITCH FOR CONTROLLING AN ELECTRICAL CIRCUIT COMPRISING AMOLDED BLOCK OF ELECTRICALLY INSULATING MATERIAL, A MEMBER MOVEABLEBETWEEN LMIT WITHIN SAID MOLDED BLOCK, SNAP-ACTION MEANS FOR POSITIONINGSAID MEMBER IN ANY ONE OF A PLURALITY OF REST POSITIONS, A MAGNETICMEMBER ON SAID MEMBER, ELECTRICAL SWITCH MEANS ENCAPSULATED WITHIN SAIDMOLDED BLOCK IN PROXIMITY TO AND OPERABLE BY SAID MAGNETIC MEMBER,ELECTRICAL TERMINALS WITHIN SAID MOLDED BLOCK, ELECTRIC CONNECTIONSIMBEDDED IN SAID MOLDED BLOCK CONNECTING SAID SWITCH MEANS WITH SAIDELECTRIC TERMINALS, AND MEANS SEALING SAID ELECTRICAL CONNECTIONS ANDELECTRICAL CONNECTIONS BETWEEN SAID TERMINALS AND AN EXTERNAL ELECTRICCIRCUIT TO BE CONTROLLED BY SAID SWITCH.